Class 9 Maths — Sets

Set: Well‑defined collection of distinct objects. Elements use \(\in\); not in uses \(\notin\).
Roster (tabular) form: A = {2,4,6,8}.
Set‑builder form: A = {x ∈ ℕ : x is even and x ≤ 10}.
Common symbols: ℕ (naturals), ℤ (integers), ℚ (rationals), ℝ (reals), Ø/∅ (empty set), U (universal set).
Equal sets: A = B if they have exactly the same elements (order doesn’t matter).

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Venn Diagram — A ∪ B and A ∩ B